Only check for for relative path on virtio devices.  Otherwise it could
break driver loading in some circumstances, notably the IceCat sandbox.

https://gitlab.freedesktop.org/mesa/drm/-/issues/39

Taken from upstream:
https://gitlab.freedesktop.org/mesa/drm/-/commit/57df07572ce45a1b60bae6fb89770388d3abd6dd

diff --git a/xf86drm.c b/xf86drm.c
--- a/xf86drm.c
+++ b/xf86drm.c
@@ -3103,15 +3103,18 @@ static int drmParseSubsystemType(int maj, int min)
     int subsystem_type;
 
     snprintf(path, sizeof(path), "/sys/dev/char/%d:%d/device", maj, min);
-    if (!realpath(path, real_path))
-        return -errno;
-    snprintf(path, sizeof(path), "%s", real_path);
 
     subsystem_type = get_subsystem_type(path);
+    /* Try to get the parent (underlying) device type */
     if (subsystem_type == DRM_BUS_VIRTIO) {
+        /* Assume virtio-pci on error */
+        if (!realpath(path, real_path))
+            return DRM_BUS_VIRTIO;
         strncat(path, "/..", PATH_MAX);
         subsystem_type = get_subsystem_type(path);
-    }
+        if (subsystem_type < 0)
+            return DRM_BUS_VIRTIO;
+     }
     return subsystem_type;
 #elif defined(__OpenBSD__) || defined(__DragonFly__) || defined(__FreeBSD__)
     return DRM_BUS_PCI;
@@ -3920,6 +3923,7 @@ process_device(drmDevicePtr *device, const char *d_name,
 
     switch (subsystem_type) {
     case DRM_BUS_PCI:
+    case DRM_BUS_VIRTIO:
         return drmProcessPciDevice(device, node, node_type, maj, min,
                                    fetch_deviceinfo, flags);
     case DRM_BUS_USB: